This invention relates generally to microwave and millimeter wave (mm-wave) radio frequency (RF) circuits, and more particularly to achieving broadband high isolation switch in Balanced Line Circuits.
FIG. 1 shows a balanced line. A balanced line 10 may be achieved by using two conductors 11 and in a symmetric environment. Such balanced lines can be achieved for example as in twisted pair cable or on insulating substrates. The input port 12 is composed of two terminal 12a and 12b. Due to symmetry, terminals 12a and 12b have opposing voltage V1 and −V1 and support equal currents 16 and 17 in opposite direction or opposing current. In a balanced line configuration because there is no other path available for the current, the forward going current has to be equal to the reverse going current at any location, for example position 18, due to charge conservation. Moreover, voltage at any location 18 along the transmission line is also equal and opposite. If the balanced line is terminated in a balanced manner (i.e., same impedance on each line) using the output port terminal 13a and 13b, the output port 13 also has opposing voltages and currents, 14 and 15, respectively, at the terminal 13a and 13b. 
Such balanced lines are widely used in substrates where ground is not easily accessible. Examples include silicon substrates without vias, which are widely used for both mm-wave and microwave frequencies.
Prior art electronic switches in balanced lines are achieved in series 20 and shunt 30 configuration, as shown in FIG. 2 and FIG. 3, respectively.
In FIG. 2, the input lines 22 and 23 have, in series, diodes 24 and 25, respectively. While diodes are depicted in this figure, in actual practice other devices that switch from a high impedance state (or blocking state) to a low impedance state (or transmitting state) may be used to perform the task. For example, the diodes could be replaced by a three terminal device, whose state is switched using one of the three terminals such as the base of a Bipolar Transistor, where the Emitter and Collector are the two ends of the switching device. In another configuration, the Emitter current is switched while the Base forms the input and the Collector the output. Considering FIG. 2, in the low impedance state when the diode is forward biased, the diodes 24 and 25 connect the input lines 22 and 23 to the output lines 26 and 27, respectively. The signal is thus transmitted in high strength. The S-parameter for the forward transmission gain, S21, is high, being close to zero decibels (dB) S-parameters, or scattering parameters, are analogous to frequency response functions, but the terms are used at high and lower frequencies, respectively. In the other state the diodes are in the non-conducting state. In that state the signal is reflected back. Now the transmitted signal to the output lines 26 and 27 is attenuated and the S21 transmission coefficient is low (−10's of dB), and is determined by the high impedance state. Since the high impedance is finite, a small amount of signal trickles through and is represented by δ1.
FIG. 3 shows a shunt mounted diode 30 in a balanced line for switch purposes. When the diode is reversed biased or is in the high impedance state, since it appears as open circuit between the lines, the signal is transmitted through or S21 is high, i.e., close to 0 dB. In the other state, diode 33 is forward biased and is in the low impedance state. In this state, because the input balanced lines 31 and 32 are effectively shorted by the small impedance, the voltage induced at the input of the balanced line 34 and 35 is effectively small. This then has very little signal transmitted to the out balanced lines 34 and 35.
In case of the series configuration 20, the impedance in the high impedance state determines the isolation. Since the impedance is finite but high impedance, a signal always leaks to the output. At mm-wave, the impedance in the high conducting state is mostly capacitive and could greatly reduce the isolation (or the magnitude of minus S21, where S21 is in dB). Similarly in the shunt configuration case the forward biased impedance or the low impedance state determines the isolation. Since the low impedance state has finite impedance (resistive at low frequency and reactive at mm-wave), the isolation is limited by this impedance.